1. Field of Art
The present invention relates to agricultural balers that produce rectangular bales and more particularly to an arrangement of a sensor which is used for controlling the density of the produced bale.
2. Description of Prior Art
In an agricultural baler, material such as hay, straw, silage or similar crop material that previously has been cut, windrowed or swathed, is picked up from the field by a pick-up unit, fed into a duct and loaded into an elongated bale chamber. A plunger which reciprocates in the front portion of the baling chamber compresses the newly introduced material against the previously introduced material into a parallelepiped package, and at the same time, gradually advances the package towards the outlet of the bale chamber. As the package reaches a predetermined length as determined by a metering device, a knotter device is actuated to wrap cord, twine or other flexible binding material around the package and to secure the ends of the binding material together to form a stable bale.
Commonly the baling chamber has at least one movable wall portion whereof the position can be changed to vary the outlet section of the baling chamber. An enlarged outlet section reduces the forces needed to advance the bales in the baler and hence diminishes the density of the newly formed bales. Otherwise, a reduced section substantially increases the friction forces of the bale along the chamber and thereby raises the compression forces exerted by the plunger on the crop material. Accordingly, the density of the new bales will increase.
It is known in the art to adjust the position of the movable wall in accordance with a signal of a load sensor in order to produce bales having a predetermined density. Typically, the position of the rear portion of the baling chamber is controlled by a hydraulic cylinder in response to a sensor monitoring the forces exerted by the plunger. U.S. Pat. No. 4,624,180 shows a baler having a plunger which is connected by a pair of wrist pins to a pair of connecting rods, which in turn are linked to a pair of rotating cranks on a gearbox. The wrist pins are equipped with strain gauges for monitoring the forces applied by the plunger. As the full compression force on the plunger is transferred upon the wrist pins, they have to be of a sturdy design. However, some deformation must be allowed to make the sensor finish discernable signals that can be used in adjusting the outlet area of the baling chamber.
Other sensor embodiments are illustrated in U.S. Pat. No. 5,123,338. According to one embodiment, a substantially mechanical sensor reacts to the forces applied by the plunger on the connecting rod. The result. is a mere ON-OFF signal which provides no intermediate data on the actual plunger load. Such output cannot be used for a refined adjustment of the bale density. The compression level achieved by this control system has to be adjusted by mechanical means on the baler itself.
Another embodiment in the same document shows a load sensor arranged between the gearbox driving the plunger and the main frame of the baler. The reaction forces of the plunger on the gearbox are measured by strain gauges affixed to an arm that is interconnecting the top of the gearbox and the frame. The output signal is variable such that it can be used for setting a wide range of densities. However, upon failure of any of the gauges, the complete assembly of arm and gauges has to be replaced, which is expensive and cumbersome.
Accordingly, there is a need for a sensor arrangement which enables easy replacement of the sensor element and which no longer requires very sturdy and relatively insensitive sensors.
According to the invention, there is provided an agricultural baler having a frame, a pick-up assembly for picking up crop material from a field and a baling chamber for receiving crop material from said pick-up assembly, said baling chamber having at least one movable wall portion. There is a plunger mounted for reciprocating movement in said baling chamber for compression of the crop material therein. The plunger being driven by a gearbox affixed to said frame. There is a control unit for adjusting the position of the movable wall portion in response to the signal from a load sensor. The load sensor senses at least a portion of the reaction forces of the crop material on the plunger during the compression stroke. The load sensor is mounted to the lever for measurement of a portion of said reaction forces.
The lever arrangement reduces the forces on the load sensor and hence enables the use of lighter and more sensitive load sensors. The lever may have its fulcrum connected to the gearbox and the arrangement may include a substantially longitudinal member connecting one arm of the lever to the frame, while the other arm is connected to the load sensor. The sensor may be connected to the frame via a linkage allowing upward movement of the gearbox. In this manner, vertical vibrations of the gearbox as caused during baling operations will have no effect on the sensor readings. To preclude the effects from sideways distortion of the gearbox or of the frame, the load sensor may be mounted to a linkage including spherical bearing means, such as a ball bearing or an assembly of a concave and a convex washer.
The load sensor preferably has a cylindrical body that is inserted transversely through the lever and the ball bearing, such that it can be replaced easily.
Advantageously, the sensor comprises a Hall effect sensor for good precision and reliable operation.